Photosensitization can result when light interacts with endogenous or exogenous chemical agents in the skin and eyes. This process can produce undesirable clinical consequences, as in phototoxicity and photoallergy; or it can have beneficial effects, as in tumor photodynamic therapy (PDT) and coal-tar or psoralen (PUVA) therapy against psoriasis. Photosensitization results from the light-induced production of free radicals and/or singlet oxygen (1O2), the lowest electronic excited state of molecular oxygen. Because the latter species may be important in both phototoxic reactions and PDT, we have developed state-of-the-art instrumentation capable of detecting the characteristic phosphorescence of 1O2 at 1268 nm. This instrumentation has permitted us to delineate the photophysics of 1O2 production from a number of photosensitizers including phenothiazines, tetracycline, benzoxazoles synthetic dyes, anthralin and 1,8-dihydroxy-anthraquinone. Singlet oxygen has been implicated in the phototoxicity of benzanthrone (7H-benz[de]anthracene-7- one), a dye intermediate prepared from 1,8-dihydroxyanthraquinone. Potential photodynamic agent 1,5-diamino-4,8-dimethoxy-anthraquinone and related compounds were shown to be efficient 1O2 generators which may explain their cytotoxicity to human leukemic cells in culture. The influence of cationic surfactants on the photoprocesses of the sensitizers eosin and rose bengal have been studied in both aqueous and organic solvents. A nano-second laser flash photolysis spectrometer has been built and successfully tested. This equipment now permits us to carry out time-resolved transient absorption and emission spectroscopy on excited state intermediates (precursors to 1O2) of photosensitizers. The spectrometer is being modified to measure singlet oxygen lifetimes in organic solvents and biological systems.